1. Field of the Invention
The present invention relates, in general, to welding and machine controls and, more specifically, to controllers for such weld and machine control operations.
2. Description of the Prior Art
Programmable logic controllers (PLC) have long been used to control various automatic machine functions, such as actuation of solenoids, pumps, motors, etc. Such PLC's utilize a control program stored in a memory within the PLC which controls the sequence of operation of the machine through its various operation steps. The PLC control program is typically in a programmed electrical relay ladder diagram form.
In a welding application, such as a spot welding operation which bonds two or more sheets of metal together, two electrodes are brought together on opposite sides of the metal sheet stack under pressure for a predetermined period of time and a high current is passed through the electrodes and the metal stack which creates a melting of the area of the metal stack between the electrodes based on the diameter of the electrode tips. This molten area, when solidified forms a nugget which bonds the metal sheets together at that particular point.
However, during repeated operations, the diameter of the electrode tips expands or mushrooms due to the forces involved in contacting the metal stack which enlarges the area through which the current passes between the electrodes and results in inadequate welds. The electrode tips must then be reduced back to their original size based on welding conditions or replaced to maintain optimum welding conditions.
Welding current control sequencers have also been devised which increase the current through the electrodes based on the number of repeated operations of the welding equipment to overcome the expansion or mushrooming of the electrodes. Such sequencers typically operated on the number of sequences or operations of the welding control and increase the current through the electrodes sequentially to overcome the expansion of the electrode tips.
Previously, controllers for activating the ignitron tube or solid state firing device have been separate from the control, either PLC or relay system, which controls the machine utilizing the welding apparatus. This has resulted in a high cost for wiring and labor due to the separate controllers.
Thus, it would be desirable to provide an apparatus which combines both weld control and machine control into one unit to reduce material and labor costs in installing and operating such equipment.